Conventional shock absorbers have a reservoir to receive hydraulic fluid displaced from the main working cylinder. However, besides acting as a space into which hydraulic fluid can be displaced from the working cylinder during reciprocating motion of the shock absorber piston, such reservoir also serves to supply hydraulic fluid for refilling the working cylinder and to make up for fluid loss due to seepage.
A volume of air is usually retained in the reservoir to allow for quick pulsing action of the fluid between the cylinder and the reservoir. The air is needed to act as a compensation space or air cushion during such pulsation. Unfortunately, such pulsation causes a high degree of turbulence in the hydraulic fluid, causing the fluid to be aerated and frothed thereby slowing the reaction time of pulsation.
One useful piece of apparatus to overcome such aeration or frothing is a deformable semi-permeable gas cell that allows air or light substances to be absorbed into the interior of the cell, resulting in a small increase in gas volume that compensates for oil seepage. Such gas cell typically is impermeable to hydraulic fluid and the enclosed gas (see U.S. Pat. Nos. 2,997,291 and 3,024,875). Freon is typically used as the gas and is introduced by needle injection, the needle opening being hot staked to form a seal. The volume of the cell is selected so that at the highest temperature expected and under a full compression stroke, in normal operation, the cell will not be fully collapsed. The deformable cell prevents aeration and keeps the hydraulic fluid under pressure at all times.
Although the principle of the flexible freon cell has been well received, its benefits have not always been realized because of difficulties associated with the installation of the cell. It is conventional to install the freon cell by the following steps: (a) an open ended outer tube is filled with oil, (b) the freon gas filled cell (usually shaped as a slim pillow formed by two sheets of nylon material) is slid into the oil filled outer tube, preferably by use of a mandrel about which the cell is wrapped, (c) the mandrel is withdrawn, (d) the inner tube assembly, including the piston, is slid into available space defined within the curled cell residing in the outer tube, and (e) the outer tube is closed, such as by welding of end assemblies.
The problems presented by such installation include:
(1) The cell, being flexible and containing a gas, does not stay in a slim, controlled, cylindrical shape after withdrawal of the mandrel. To the contrary, the cell, as a result of gas pressure, billows out and is squeezed during the insertion step of the inner tube assembly. The design of the space receiving the cell in the fully assembled device is narrow by necessity. Gas is thus forced to one end of the single cell and the remainder of the cell is collapsed. This condition is further complicated by the unrefrigerated storage of the cell prior to installation whereby it is allowed to absorb additional air from the surrounding environment, exaggerating the forcing action.
(2) The manner in which the gas containing cell must be inserted into the oil filled tube is slow and awkward, resulting in low productivity and requiring an excessive amount of workmen to carry out the assembly task. Thus, the normally intended virtues for such freon cell are not fully realized and can cause less than satisfactory shock absorber operation.